In recent years, high-density magnetic recording media obtained by coating the surface of a substrate with a hexagonal ferrite magnetic powder having uniaxitial magnetocrystalline anisotropy have attracted a great deal of attention.
There is an advance in development, in particular, of magnetic recording media with a Co-substituted barium ferrite magnetic powder applied to the surface of a substrate, for example, of floppy disks, hard disks and magnetic tapes. The Co-substituted barium ferrite magnetic powder can be obtained by substituting Co for part of Fe, thereby reducing the coercive force (in general, about 5,000 Oe) inherent in barium ferrite to a coercive force (200-3,000 Oe) suitable for use as magnetic recording media.
By the way, when industrially mass-producing magnetic recording media, the variation tolerance in coercive force of an intended magnetic recording medium is required to be within (the preset value .+-.20) Oe from the necessity of keeping the quality of various characteristics and properties, including electromagnetic conversion characteristics, constant.
In order to control the coercive force of a magnetic recording medium with such good precision, the following conditions are required. Namely, (1) the coercive force of a magnetic powder used is stable and keeps a constant value and (2) when a magnetic coating formulation containing this magnetic powder is dispersed and coated on the surface of a substrate to form a magnetic recording medium, a coercive force forecast is not varied.
However, it has been known that with respect to conventionally known Co-substituted barium ferrite magnetic powders containing Co in a great amount, their coercive forces are varied due to agglomeration caused by magnetostatic interaction between particles of the magnetic powders [Yokoyama, et al., Journal of the Magnetics Society of Japan, Vol. 13, No. 3, pp. 488-489 (1989)].
Namely, as indicated in the simultaneously rotating model of the Stoner-Wohlfarth's magnetization, ultrafine particles of the barium ferrite are susceptible to the magnetostatic interaction between the particles of the magnetic powder because the crystalline anisotropic magnetic field (H.sub.A) and the anisotropic magnetic field (H.sub.K) of the magnetic particles are decreased to a great extent by the substitution of Co. Therefore, the coalescence and agglomeration of the magnetic particles tend to occur and the coercive force varies in a wide range depending upon their coalescent states.
In deed, as shown in Table 1, the conventionally known Co-substituted barium ferrite magnetic powders have the same composition, but their coercive forces are different from one another in a wide range.
In addition, the present inventor was found that when Co-substituted barium ferrite magnetic powders are treated for the purpose of adjusting their water contents or improving their dispersibility, their coercive forces vary greatly depending on the states of the magnetic powders after treatment even in those of the same preparation lot (see Comparative Example 1 which will be described subsequently).
As apparent from these facts, the coercive forces of the known Co-substituted barium ferrite magnetic powders proposed to use as magnetic recording media and containing a relatively great amount of Co vary depending on their coalescent states and the like. It is therefore impossible to satisfy the above-mentioned requirement that the coercive force is stable and keeps a constant value.
TABLE 1 ______________________________________ Magnetic properties Example Barium ferrite Hc .sigma.s JPO No. composition (Oe) (emu/g) ______________________________________ 149328/81 Example Ba.sub.1.2 Fe.sub.10.4 Co.sub.0.8 Ti.sub.0.8 870 47 1 60002/81 Example Ba.sub.1.2 Fe.sub.10.4 Co.sub.0.8 Ti.sub.0.8 950 58 56326/82 Example Ba.sub.1.2 Fe.sub.10.4 Co.sub.0.8 Ti.sub.0.8 1300 60 95902/85 Example Ba.sub.1.2 Fe.sub.10.4 Co.sub.0.8 Ti.sub.0.8 680 39 2 ______________________________________ Note: JPO stands for Japanese Patent Application LaidOpen No.
In addition, magnetic recording media making separate use of the conventionally known Co-containing hexagonal barium ferrite magnetic powders are accompanied by a problem that the coercive forces of the magnetic powders used vary greatly from those of the magnetic recording media formed therefrom.
For example, as shown in Table 2, the coercive forces of the magnetic recording media increase to about at least 1.1 times, and sometimes, about 2.0 times the coercive forces of their corresponding Co-containing hexagonal barium ferrite magnetic powders used and moreover, since the degree of variation in coercive force differs with compositions, it is impossible to accurately forecast a relation between the coercive force of each of the magnetic powders and the coercive force of the magnetic recording medium produced therefrom. It is hence very difficult to satisfy the above-mentioned requirement that when a magnetic coating formulation containing a magnetic powder is dispersed and coated on the surface of a substrate to form a magnetic recording medium, a coercive force forecast is not varied.
For this reason, it is difficult to industrially mass-produce magnetic recording media having a fixed quality from any Co-containing barium ferrite magnetic powders by any conventional methods.
TABLE 2 __________________________________________________________________________ Co-containing hexagonal Coercive force Coercive force of Hc of Tape/ barium ferrite magnetic of magnetic magnetic recording Hc of magnetic JPO powder powder, Hc (Oe) medium, Hc (Oe) powder __________________________________________________________________________ 157718/85 Ba.sub.1.0 Fe.sub.8.0 Co.sub.0.40 1110 1210 1.09 168532/86 Ba.sub.1.0 Fe.sub.10.4 Co.sub.0.80 Ti.sub.0.80 845 925 1.09 207720/87 Ba.sub.1.0 Fe.sub.5.73 Co.sub.0.42 Ti.sub.0.42 Si.sub.0.1 515 652 1.27 216922/87 Ba.sub.1.0 Fe.sub.10.2 Co.sub.0.9 Ti.sub.0.45 Sn.sub.0.45 550 800 1.45 235220/87 Ba.sub.1.0 Fe.sub.6.93 Co.sub.0.53 Ti.sub.0.53 1310 1450 1.11 64626/88 Co-substituted barium ferrite 550 620 1.13 __________________________________________________________________________ Note: JPO stands for Japanese Patent Application LaidOpen No.
Further, the above-cited literature [Yokoyama, et al., Journal of the Magnetics Society of Japan, Vol. 13, No. 3, pp. 488-489(1989)]suggests that the coercive force of barium ferrite magnetic powder varies depending upon the coalescent state (dispersibility) of the magnetic powder even when using any substituting elements without limiting to Co so long as they are elements capable of reducing the coercive force of the magnetic powder. In other words, it suggests that any barium ferrite magnetic powders having a coercive force reduced to a value suitable for use as magnetic recording media are varied in coercive force depending upon their coalescent states.
Accordingly, it is very difficult to industrially mass-produce magnetic recording media having the same quality from any substituted hexagonal ferrite magnetic powders, whose coercive forces have been reduced, by the conventional methods.